1. Field of the Invention
The invention relates to an apparatus and process for removing impurities, and more particularly, to an apparatus and process for removing fine impurities from a fibrous suspension.
2. Discussion of Background Information
Fibrous suspensions, particularly those obtained from recycled paper which are intended for paper production, contain undesirable attendant materials, also referred to as "interfering materials," which must be removed in the preparation system. The removal of these interfering materials is generally performed in various steps since a fibrous suspension purification in a single process step is rarely successful.
Wet straining processes have turned out to be particularly effective for the removal of fine impurities. In such processes, the suspension is conveyed into a straining device and is divided into at least two portions, an "accepts" portion, i.e., the materials that have passed the wire, and a "rejects" portion, i.e., the materials rejected due to their size. Generally, in the parlance of paper makers, such wet straining processes are known as "sorting." It is also known to subject the rejected materials, i.e., the rejects portion, to one or more additional subsequent straining steps. This process is known as "multistep sorting." Multistep sorting can be carried out at various points of the paper stock preparation. Generally, the later the sorting occurs during the process, the finer the wire openings that can be used in the process.
Examples of multistep wet straining are demonstrated in the scientific paper "Sortierung von Altpapierstoff zur Herstellung von graphischen Papieren" [Sorting of Recycled Paper Stock for the Manufacturing of Graphic Papers] by R. Rienecker from the Wochenblatt fir Papierfabrikation [Weekly for Paper Making] No. 23/24, 1997, pp. 1149 to 1159.
Although a large number of different systems are already known and used for purifying paper stock suspensions, these systems are often unsuccessful in removing all interfering impurities from the suspension. In particular, very fine, adhesive impurities, which often cause great difficulties in the production of paper, even in small quantities, can only be removed at great expense, if at all.